text: Biological processes can employ cyanide-like umpolung reactivity without having to rely on the toxic cyanide ion. Thiamine (which itself is an N-heterocyclic carbene) pyrophosphate (TPP) serves a functionally identical role. The thiazolium ring in TPP is deprotonated within the hydrophobic core of the enzyme, resulting in a carbene which is capable of umpolung. 
  Enzymes which use TPP as a cofactor can catalyze umpolung reactivity, such as the decarboxylation of pyruvate.
  In the absence of TPP, the decarboxylation of pyruvate would result in the placement of a negative charge on the carbonyl carbon, which would run counter to the normal polarization of the carbon-oxygen double bond.